There is a need for anisotropic optical films that demonstrate negative optical retardation dispersion. For example, a quarter wave film made with negative dispersion birefringent materials will be largely achromatic. Devices such as reflective LCDs that utilise such a quarter wave film will have a dark state that is not coloured. Currently such devices have to use two retarder films to achieve this effect.
The dispersive power of such a negative dispersion birefringent film can be defined in many ways, however one common way is to measure the optical retardation at 450 nm and divide this by the optical retardation measured at 550 nm (R450/R550). If the on-axis retardation of a negative retardation dispersion film at 550 nm is 137.5 nm and the R450/R550 value is 0.82, then such a film will be largely a quarter wave film for all wavelengths of visible light and a liquid crystal display device (LCD) using this film as, for example, a circular polarizer would have a substantially black appearance. On the other hand, a film made with an on axis of 137.5 nm which had normal positive dispersion (typically R450/R550=1.13) would only be a quarter wave film for one wavelength (550 nm), and an LCD device using this film as, for example, a circular polarizer would have a purple appearance. Another way of representing this information is to plot the change in birefringence as a function of wavelength. FIG. 1 shows a typical birefringence against wavelength plot for a polymerized film made from the commercially available reactive mesogen RM257 (Merck KgaA, Darmstadt, Germany). The R450/R550 for this compound is around 1.115.
In an anisotropic optical film formed by rod-shaped, optically anisotropic molecules, the origin of the retardation dispersion is due to the fact that the two refractive indices ne, no, of the anisotropic molecules (wherein ne is the “extraordinary refractive index” in the direction parallel to the long molecular axis, and no is the “ordinary refractive index” in the directions perpendicular to the long molecular axis) are changing with wavelength at different rates, with ne changing more rapidly than no towards the blue end of the visible wavelength spectrum. One way of preparing material with low or negative retardation dispersion is to design molecules with increased no dispersion and decreased ne dispersion. This is schematically shown in FIG. 2. Such an approach has been demonstrated in prior art to give LC's with negative birefringence and positive dispersion as well as compounds with positive birefringence and negative dispersion.
If the compounds are polymerizable, or are mixed with a polymerizable host material comprising for example polymerizable mesogenic compounds (also known as “reactive mesogens” or “RMs”), it is possible to prepare anisotropic optical polymer films with negative dispersion. This can easily be carried out by in situ polymerization, e.g. by exposure to heat or UV radiation, of the polymerizable material when being uniformly oriented in its mesophase, thereby permanently fixing the macroscopically uniform orientation. Suitable polymerization methods are well-known to the person skilled in the art, and are described in the literature.
Prior art describes the use of coatable materials having negative dispersion for the preparation of optical films. For example, JP 2005-208146 A1 and WO 2006/052001 A1 disclose polymerisable materials largely based on compounds with a “cardo” core group for the preparation of polymer films with negative dispersion.
Prior art also reports that optical films with negative dispersion can be obtained by stretching solvent cast or extruded polymer films. For example, A. Uchiyama and T. Yatabe in Journal of Polymer Science, Part B: Polymer Physics, Vol. 41, 1554-1562 (2003), and Jpn. J. Appl. Phys., 42, 5665-5669 (2003), and WO 00/26705 A1 disclose stretched optical polymer films with negative dispersion. These documents report the use of a “cardo” group to introduce some high refractive index component in a direction orthogonal to the stretched direction of the polymer.
Other documents disclose that the optical properties of stretched polymer films can be controlled by combining polymer components with intrinsically positive and negative birefringence. This optical effect can be achieved either by combining two miscible polymer to make a polymer blend (see for example H. Saito and T. Inoue in Journal of Polymer Science, Part B: Polymer Physics, Vol. 25, 1629-1636 (1987); or K. Kuboyama, T. Kuroda and T. Ougizawa in Macromol. Symp. 249-250, 641-646, (2007)), or by combining two polymer parts or segments; one segment intrinsically positive and the other segment intrinsically negative birefringence into one homopolymer (see for example A. Uchiyama and T. Yatabe in Jpn. J. Appl. Phys. Vol 42, 6941-6945 (2003)).
The terms “intrinsically positive” and “intrinsically negative” birefringence are used to describe the optical properties of polymer films. Examples of stretched polymer films with intrinsically positive birefringence include the following: polycarbonates, polyarylates, polyethylene teraphthalate, polyether sulphone, polyphenylene sulphide, polyphenylene oxide, polyallyl sulphone, polyamide-imides, polyimides, polyolefins, polyvinyl chloride, cellulose. Examples of stretched polymer films with intrinsically negative birefringence include the following: styrene, acrylic ester polymers, methacrylic ester polymers, acrylonitrile polymers.
The intrinsic birefringence is defined as follows:(Δnint)=(ne−no)wherein ne and no are the extraordinary and ordinary index of the polymer molecular chain, respectively.
The birefringence of the polymer film depends on two basic factors: firstly the various processes used to prepare the film, like for example casting, annealing and stretching, which determine the state of the polymer backbone, and secondly, the intrinsic birefringence of the polymeric material. The magnitude and sign of the latter depends on the polarisability of functional groups and the arrangement of these groups relative to the main chain. For example, polystyrene has an aliphatic backbone with more polarisable phenyl groups orientated largely orthogonal to it. Similarly, polyacrylonitrile has an aliphatic backbone and highly polarisable nitrile group oriented largely orthogonal to the backbone. Both the later polymers demonstrate negative intrinsic birefringence. In other words, the sign of the intrinsic birefringence is determined by the orientation of the chromophore relative to the aligned polymer backbone. For intrinsically negative birefringent polymers, the transition moment of the polarisable groups or chromophores should be oriented away from the polymer backbone. This is further discussed in WO 2007/075264 A1.
Methods of measuring the birefringence of a cast polymer film are described by J. S. Machell, J. Greener, and B. A. Contestable in Macromolecules, Vol. 23, No. 1, (1990).
However, many of the materials disclosed in the literature have drawbacks, like for example, the difference between the polarisability of the groups orientated parallel and perpendicular to the stretch axis of the polymer is not sufficiently large, so the polymeric material has to contain a relatively high concentration of units that lower the birefringence dispersion, which are normally the expensive component of the total pre-polymer mixture.
Therefore, there is a still a need for materials that are suitable for the preparation of polymer films with negative optical dispersion, which are easy to synthesize and available at reduced cost, have good proccessability and have improved properties such as solubility and thermal properties.
It is therefore an aim of the present invention to provide novel and improved materials for use in polymer films with negative optical dispersion, which show the above-mentioned advantageous properties and do not have the drawbacks of the prior art materials. Another aim of the invention is to extend the pool of materials and polymer films having negative dispersion that are available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing compounds, materials and films as claimed in the present invention.
In particular, it was found that this can be achieved by using axially substituted cyclohexane diols to prepare efficient segments in copolymers that can be processed into optical films showing negative dispersion.
To achieve negative birefringence dispersion in a stretched polymer film, the wavelength dispersion of the ordinary refractive index should be greater than the wavelength dispersion of the extraordinary refractive index. Intrinsically negative birefringent polymers or segment of polymers enable the wavelength dispersion of the ordinary refractive indices of the polymer to increase. Therefore, a polymer film that demonstrates negative birefringence dispersion should be composed of a suitable molecular ratio of intrinsically negative and intrinsically positive birefringent segments. It is advantageous to minimise the amount of intrinsically negative birefringent segments in the polymer because it decreases the birefringence of the stretched film. It is reasonable to assume that to maximise the “efficiency” of the polymer segment that contributes to lowering the dispersion of the film, a segment should possess highly polarisable groups and that the polarisable groups should be oriented largely orthogonal to the main chain, and furthermore, its orientation should be constrained so that it remained in this preferred orientation.